Cheers all.
I've had a scoot on the internet. Seems to be a few options. thecaravanwarehouse.co.uk seems to only sell the exact same mattress that I have now. They even state that it's what is supplied to new Willerby, ABI etc. Starts at £100!!

I am going to try to find some with a foam memory layer and a bit firmer than what I have.
The other option available to me is a spare memory foam mattress. My dad has a spare that he no longer wants. Hardly ever been used. Has anyone had any experience in chopping one of these up to fit? I could probably get 2 single out of it. Just not sure it'll stay all in one piece or just crumble into bits once I take a Stanley knife to it.
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